The present invention relates generally to machine detectable security markings. More specifically, the present invention relates to security articles having light emission characteristics that change in response to small changes in temperature, whether heating or cooling.
Counterfeiting and forgery have become significant concerns in the modem economy and marketplace. Advances in computing technology and printing techniques have increased the incidence of forgeries, counterfeited documents, and other fraudulent activities. Countless areas of today's high-technology society require and rely upon certification, authentication and protection of highly valuable documents, papers, currency and other materials. Thus, there is a need for security markings to be incorporated into currency, important documents, packaging, and other authentic materials to prevent unauthorized copying, forging, counterfeiting and other fraudulent use.
A similar problem exists in a variety of contexts. In addition to protecting against counterfeit currency, authentication of valuable documents or materials also affects many facets of the economy. Authentication stamps such as visas or postmarks, for example are subject to fraudulent use and forgery. Also a wide variety of products and consumer goods may be created as knock-offs cheap replacements or gray market goods. Notaries public use a raised stamp to authenticate notarized documents. Drivers' licenses, passports and other photographic identification contain holograms and microprinting. Sporting memorabilia and retail clothiers use holographic tags and stamps to assist in proving authenticity. Even fashion designers are now including authentication devices in their clothing to prevent passing off of knock-offs as designer products.
Current methods of authentication of currency involve visual observation, scanning under ultraviolet lamps, notes containing security threads, and emissive materials such as inks and planchettes. Such security threads emit a distinct marking, color or code in response to exposure to the ultraviolet light. In some circumstances, the emissive features of different denominations of notes can emit different colors. In addition to the colors of the emission, a code number or other unique identifier can be detected by the naked eye when the note is exposed to ultraviolet light or excitation of some form.
A disadvantage to most of the traditional security features is that they are visible and known to the world. If a counterfeiter is aware there is a security thread in a bill or a watermark in a document, replication of the security feature is easier. Once a feature is made known to the public, a counterfeiter may begin to develop specific strategies and solutions to overcome the security protections provided by the specific feature. Methods of creating and perfecting forgeries and counterfeit documents have become easier and more available with the advent of highly sophisticated computer printing and processing. As far back as 1991, the United States Treasury has continually added security safeguard features to the denominations of currency in an attempt to combat the use of counterfeit money. These safeguards have included watermarks, security threads embedded in the paper, microprinting, color-shifting ink, and the use of multi-colored bills.
Accordingly, a need exists for security features that further reduce a counterfeiter's likelihood of success, even if they are aware of the existence of the security feature. There is a further need for covert optically encoded markings, formed from at least one emitter that responds differently when exposed to two varied stimuli to create a machine readable response that cannot be detected by the human eye.